Mega Bangna is a huge shopping mall in Samutprakarn Province. Even if it is not located in Bangkok, it is convenient to visit. And, I always feel like i am still in Bangkok when I am there because it is only 30 minute-drive from my home.
You can read my review about Mega Bangna here.
If you do not have a private car, no worries. Mega Bangna provides free shuttle bus service for their customers. The bus is at BTS Udomsuk. After getting off the BTS, take exit 5. Then, you will see people waiting for the bus. The free shuttle bus runs daily from 8.00 to 23.00 and there are more buses on weekend. You can see the timetable here.